Some words on boosters
Boosters should just raise the volume of your signal. Simply put: make it louder. As long as there's headroom available, the volume boost will be just that, more loudness. No headroom and you enter distortion.
It can be a memorable "aha" moment (it was for me) when you discover the relation there is between the cleanness of your amp and the output volume of your distortion pedals. Try a fuzz pedal into an already overdriven amp and half the range of the level knob becomes useless, you'll hardly get it any louder; set the amp clean and try again… So don't discard incorporating your amp's knob into your tweaking when experimenting and searching for sounds.
Besides the so-called clean boosts there's also treble boosters, and boosters with EQ options (the Xotic RC booster is a good example) which can help you achieve different kinds of overdrive and grit, by pushing your amp (or pedals) only in certain frequencies.
The way a booster pedal should be
Of the single-knob boosters available on the market there's only one I know that can not only boost your signal but also attenuate it, doubling the pedal's functionality: the Gravinton boost from Field Effects by Resonant Electronic.
Although only a bit more than half of the knob is available actually for boosting, the range is smooth and feels very natural. The Graviton also offers you a better version of your sound at unity gain (I believe that's due to high input impedance, which rescues a part of your guitar's signal that usually gets lost — I learned that by building Jack Orman's Mosfet Booster.)
Transparent, with the ability to both boost and cut… Here's a short clip for illustration:
The new Blowing Up has mercy
The booster Brooklyn-based Ohnoho introduced in 2010 recently got updated. A meant to be weapon of destruction is now an extremely versatile tool. The single gain knob has been replaced by a high/low switch and a master volume knob. It will make stuff blowing up the same as deliver sweet grit.
The new Blowing Up sounds like this:
Further reading: Headroom (ovnilab.com)